1. Field of the Invention
This invention relates to an external power source on/off control system, and more particularly, to an external power source on/off control system which is incorporated in an interface cable for electrically connecting a host apparatus to a terminal apparatus.
2. Description of the Related Art
In an interface cable for electrically connecting a host apparatus to a terminal apparatus, such as a bar-code reader, a printer, a display, a keyboard or the like, an interface cable including therein power supply lines as well as signal lines is known. Such an interface cable is conventionally used in the terminal side of the POS (Point of Sale) system. Of course, an interface cable including power supply lines as well as signal lines is used for supplying electric power from an internal power source in the host apparatus to the terminal apparatus, as well as for signal or data communication between the host and the terminal apparatus.
Regarding such POS systems, it is expected that the number of the terminal apparatuses will be increased with respect to a single host apparatus. Thus, a large number of input/output connectors for the interface cables must be prepared. However, since the capacity of the internal power source of the host apparatus is limited, the necessary power for all of the terminal apparatuses will be larger than the capacity of the internal power source. In this case, an external power source, such as an AC adaptor, must be used to supply electrical power to the respective terminal apparatus.
Therefore, the terminal apparatus is conventionally provided with connectors for an external power supply, to which a connector extending from the external power source can detachably be connected. Of course, the terminal apparatus is connected to the host apparatus by means of an interface cable, by which the communication of control signals and data between the host and terminal apparatuses can be conducted. In this case, however, the power supply line in the interface cable should be disconnected from the power supply line of the host apparatus.
As mentioned above, if the external power sources are appropriately used, it becomes possible to increase the number of terminal apparatuses in spite of the limited capacity of the internal power source of the host apparatus. However, in the terminal apparatus using an external power source, when the internal power source of the host apparatus is turned off, the external power source of the terminal apparatus is not automatically turned off, but usually continues to be turned on. Thus, in this case, the switch of the external power source of the terminal apparatus should usually be manually turned off by the operator, but sometimes this is forgotten. If the number of terminal apparatuses is increased, the probability that the operator may forget to turn off the power switches of the terminal apparatuses will also increase.